05.24.09 6:01 AM ET

Biological evidence on a knife, a bra clasp, a stained Q-Tip box, and in five other locations throughout the house were the focus of testimony Friday and Saturday as the prosecution began to wrap up its case against Amanda Knox, who is accused of murdering roommate Meredith Kercher in Perugia, Italy. Forensic biologist Patrizia Stefanoni testified for nine grueling hours on Friday and took the stand again Saturday to explain how this crucial evidence ties both Knox and her former Italian boyfriend Raffaele Sollecito to the crime.

Investigators noticed peculiar diagonal scrapes on the side of the blade seemed to suggest someone had vigorously rubbed the knife, perhaps to clean it.

Stefanoni, who was responsible for collecting forensic evidence at the murder scene, testified that a 20-inch kitchen knife with Amanda Knox’s DNA on the handle and Meredith Kercher’s genetic material on the blade is the likely murder weapon. The knife, which is consistent with one of three wounds on Kercher’s neck, was found in Sollecito’s apartment. Investigators noticed it there because of some peculiar diagonal scrapes on the side of the blade, said Stefanoni, that seemed to suggest someone had vigorously rubbed the knife, perhaps to clean it.

On cross examination, defense attorneys for both Knox and Sollecito tried to discredit the knife as evidence. Knox attorney Carlo Dalla Vedova pointed out that it was stored in a box not supplied by the forensic team, that it might well have just been a knife that Knox used to cut bread, and that the genenic sample on the knife that matches Kercher was so small there was no way to double-check the results. “If the blood evidence is a positive match, it is not always important how much there is,” Stefanoni said. “And the material on the blade matches the victim.”

But perhaps more damning even than the knife was Stefanoni’s testimony that a mix of Knox’s DNA and Kercher’s blood was found on the floor in the bedroom of a third roommate, Filomena Romanelli. While it might not be noteworthy to find mixed genetic traces of residents of the same house, Romanelli’s room is critical in this crime. Her window was broken with a large rock that prosecutors believe was used to stage a break-in. The mixed Knox-Kercher trace was found after investigators used luminol, a substance used in forensic science to bring out blood that had been cleaned up.

In addition, Stefanoni testified that a mixture of Knox’s DNA and Kercher’s blood was found on the drain of the bidet, on the bathroom sink and on a Q-Tip box in the girls' bathroom. Stefanoni also found Kercher’s blood on the upper part of the light switch in that room, which indicates that it was left when the light was turned on, not off. Two weeks ago, the court heard that a nude footprint in Kercher’s blood found on a blue bathmat was likely Sollecito’s, and Stefanoni further testified that Sollecito’s DNA was found on the clasp of the bra that had been cut off Kercher’s body during the assault.

Defense attorneys maintain that all these seemingly incriminating forensic elements can be explained as contamination due to the sloppy police work of Stefanoni and her colleagues. Sollecito’s attorney, Giulia Bongiorno, stopped the crime scene video several times to point out errors. For example, Stefanoni testified that she had changed gloves according to official investigation procedures, but Bongiorno stopped the crime scene video twice to show that Stefanoni’s bracelet and the fold of her glove were exactly the same before and after the time she claimed to have changed gloves.

In another segment of the video, defense lawyers insisted that one of the officers used the same collection swab to swipe two different blood samples, but Stefanoni quickly explained that even if she had, it would not matter. “If the same cloth was used to collect two samples of different blood it would be evident in the data analysis," she said. “These samples match Knox and the victim.”

On Saturday, Francesco Camana, an expert with Rome's criminal forensic division, testified that based on the trajectory of blood splatters, Kercher was most likely killed while on her knees facing her wardrobe. He will be cross-examined on May 29. Outside the courtroom, Chris Mellas, Knox’s stepfather, who is taking his turn in the family tag team that stays in Perugia to support Amanda, said of Friday’s testimony, “I think it went OK. When you look at all the court dates, we have only had one, maybe two iffy days.” But those might just be the days that count.

Barbie Nadeau has reported from Italy for Newsweek Magazine since 1997. She also writes for CNN Traveller, Budget Travel Magazine and Frommer's.